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Got a question for all of you. I just got my new bike and it came with 172.5 mm cranks. I'm 160 lbs, 5'11 with an inseam of 876.3 mm (35.5"), my previous cranks were 175mm and my mountain bike cranks are 175mm. What length is correct for me? Would a 172.5 mm work for me? All the research I've been doing says that I should be riding a 180 mm lenght crank, but it also says that a shorter crank will be better for spining. Will a 172.5 be too short for me? Thanks

Don't know how you came up with 180mm. No one uses that unless your doing a time trial, or super tall. 172.5mm is probley perfect for you, especially if your more on the spinning side. If your a power house and like jamming low rpms, then 175mm at most.

I'm not an expert on this, but I think it's mostly preference. I'm 5'10 and ride 172.5 and they work great! But it is true that the shorter the length, the faster the spin but the longer ones give more leverage. So it's preferance for your type of riding. If you like pushing bigger gears, the longer ones will make it easier. Correct me if I'm wrong on this guys....

Lets put our differences aside here for a minute , but I totally disagree with your coach. In specific events different crank lenghts is disered. Like in time trials, many riders in the tour increase there crank lenght any where from 2.5- 7.5 mm for that extra power to the wheels for more speed.

Plus mountain bike crank length tends to be one size bigger then road size, bit not always the case.

Superlite wrote:Lets put our differences aside here for a minute , but I totally disagree with your coach. In specific events different crank lenghts is disered. Like in time trials, many riders in the tour increase there crank lenght any where from 2.5- 7.5 mm for that extra power to the wheels for more speed.

Plus mountain bike crank length tends to be one size bigger then road size, bit not always the case.

i have no differences...

if you read what i posted i did say were possible, my coach is a multi world champion & has coached the british track sqaud in the past so knows his stuff...

obviously when you start looking @ track riders etc then its not always possible to use the same lengh cranks

I don't know what my optimum or average cadence is with the 175's. Don't have a cadence monitor...yet! My main concern is the change from 172.5 to the 175's on my mountain bike. I mostly do mountain bike racing, but this season I want to do more road racing. With that being said, I don't want to go a step backwards, by riding a shorter crank that will be a detriment instead of an improvement for both my mountain biking and road riding. I have heard both sides of the arguments you guys brought up, keeping the same length and changing lengths according to the type of riding. So I wanted to hear what most of you are riding and what you guys recommend. Thanks

Your not going backwords, remember mountain bikes have different gearing and wheel size the road bikes, the two don't directly correlate with each other. Like I said, it's not unusual to have a size difference, one size up from road to your mountain bike.

Any signifigent change in crank length is noticable to a trained cyclist. 2.5mm is noticable to some, but not others, and 5mm is to much for me.

The stories of people who complain about the lack of ability to 'spin' on longer cranks generally have not given themselves time to adapt. Going form 170 to 180mm cranks took me some time to get used to, but once adapted I was able to return to a high cadence in track sprints (180-200rpm).

Jumping on a MtB for the first time it had standard 175s and on these I kept trying to find the extra 5mm till I got some 180s.

i made the mistake of using different crank lengths over the season just gone, i went from 170's to 175's & it really messed my cadence up.... i have now started training again on 175's & will not be using anything else from now on....

I disagree with the people that say it depends on your style. It only depends on your bike fit. It is true that if you use a longer crank you will get more leverage. Take for example Lance, in this years tour he used 172.5mm cranks on his timetrial bike but he used 175mm on his road bike. Were talking about a guy who like to spin so why can he spin 90+ rpm on a climb the day after a tt where he used 172.5mm cranks. Shouldnt he a well trained athlete notice a difference? The reason why pro's use different length cranks on different bikes isnt to gain more power its to get the correct fit. Name me someone that has the EXACT same bike fit on their time trial bike their road bike and their mtn bike. Nobody does because each of those bikes usually has different seat tube angles. Peter you should if you havent already is find someone that knows how to do a professional bike fit. After that you should measure your femur length to find your optimum crank length. the longer crank for timetrialing is a myth. You need to really just see what works for you. Because your not the same as anybody on this forum.

First...yu got your new bike and u didnt order it with your normal crank lenght ...weirdo..

Anyway...it doesnt matter actually. If yu use the same power in your legs with the longer cranks, yu going to have more Watt and power on the street. For example on climbs yu can use a shorter gear; lets say 19 instead of 21. That sounds good and it is till yu reach your biomechanical limit. The energy yu need to spin the cranks with the same cadence like the little ones, will soak up the energy yu gain with the length.
This effect yu wont recognize on flat terrain but yu will when the cadence drops. If the mountain is steep yu get in trouble when the cadence drops under 70. Its hard to rotate the cranks.

So...the more power yu got in your legs the less important is the cranklenght. Thats why all MTbs got big cranks ...LOL

MiNtzNUrMth makes a good point about knowing the optimal relationship between leg structure and crank length. I did a search in the scientific literature.. and here were the most relevant hits (listed below with abstracts). I've read a few of these papers, and I can say that this questions is still up for debate. I think the provailing wisdom of muscular adaption to a specific crank length is more important then length itself. Personally, I've opted for the longest crank I can comfortably use, 172.5mm. This is in contrast to the Wobblenaught professional bike fit system, which measured my leg bones to the mm, and suggested through a blanket equation a 165 or 170mm crank. My muscles have adapted to the 172.5, and I know this helps me in the mountains. I used to race on 165, 170, and 171mm cranks for years, and I can notice the difference.

I will forward this question to a friend of mine who is athletic biomechanic Maybe we can get some poor student to earn their PhD on this subject. For all I know, someone could be already working on it. Just realize that this is a VERY complex question, as the number of variables is huge. Is not a simple ratio of bone length to crank length, if it were, it would have been figured out a long time ago, right!?!

This study exploited the alterations in pedal speed and joints kinematics elicited by changing crank length (CL) to test how altered task mechanics during cycling will modulate the muscle activation characteristics in human rectus femoris (RF), biceps femoris long head (BF), soleus (SOL) and tibialis anterior (TA). Kinetic (torque), kinematic (joint angle) and muscle activity (EMG) data were recorded simultaneously from both legs of 10 healthy adults (aged 20-38 years) during steady-state cycling at 60 rpm and 90-100 W with three symmetrical CLs (155 mm, 175 mm and 195 mm). The CL elongation (DeltaCL) resulted in similar increases in the knee joint angles and angular velocities during extension and flexion, whilst the ankle joint kinematics was significantly influenced only during extension. DeltaCL resulted in significantly reduced amplitude and prolonged duration of BF EMG, increased mean SOL and TA EMG amplitudes, and shortened SOL activity time. RF activation parameters and TA activity duration were not significantly affected by DeltaCL. Thus total SOL and RF EMG activities were similar with different CLs, presumably enabling steady power output during extension. Higher pedal speeds demand an increased total TA EMG activity and decreased total BF activity to propel the leg through flexion into extension with a greater degree of control over joint stability. We concluded that the proprioceptive information about the changes in the cycling kinematics is used by central neural structures to adapt the activation parameters of the individual muscles to the kinetic demands of the ongoing movement, depending on their biomechanical function.

The mechanical efficiency of cycling with a new pedal-crank prototype (PP) was investigated during an incremental test on a stationary cycloergometer. The efficiency values were compared with those obtained, in the same experimental conditions and with the same subjects, by using a standard pedal-crank system (SP). The main feature of this prototype is that its pedal-crank length changes as a function of the crank angle being maximal during the pushing phase and minimal during the recovery one. This variability was expected to lead to a decrease in the energy requirement of cycling since, for any given thrust, the torque exerted by the pushing leg is increased while the counter-torque exerted by the contra-lateral one is decreased. Whereas no significant differences were found between the two pedal-cranks at low exercise intensities (w*=50-200 W), at 250-300 W the oxygen uptake (V*O2, W) was found to be significantly lower and the efficiency (eta=w*/V*O2) about 2% larger (p<0.05, Wilcoxon test) in the case of PP. Even if the measured difference in efficiency was rather small, it can be calculated that an athlete riding a bicycle equipped with the patented pedal-crank could improve his 1h record by about 1 km.

The metabolic cost of producing submaximal cycling power has been reported to vary with pedaling rate. Pedaling rate, however, governs two physiological phenomena known to influence metabolic cost and efficiency: muscle shortening velocity and the frequency of muscle activation and relaxation. The purpose of this investigation was to determine the relative influence of those two phenomena on metabolic cost during submaximal cycling. Nine trained male cyclists performed submaximal cycling at power outputs intended to elicit 30, 60, and 90% of their individual lactate threshold at four pedaling rates (40, 60, 80, 100 rpm) with three different crank lengths (145, 170, and 195 mm). The combination of four pedaling rates and three crank lengths produced 12 pedal speeds ranging from 0.61 to 2.04 m/s. Metabolic cost was determined by indirect calorimetery, and power output and pedaling rate were recorded. A stepwise multiple linear regression procedure selected mechanical power output, pedal speed, and pedal speed squared as the main determinants of metabolic cost (R(2) = 0.99 +/- 0.01). Neither pedaling rate nor crank length significantly contributed to the regression model. The cost of unloaded cycling and delta efficiency were 150 metabolic watts and 24.7%, respectively, when data from all crank lengths and pedal speeds were included in a regression. Those values increased with increasing pedal speed and ranged from a low of 73 +/- 7 metabolic watts and 22.1 +/- 0.3% (145-mm cranks, 40 rpm) to a high of 297 +/- 23 metabolic watts and 26.6 +/- 0.7% (195-mm cranks, 100 rpm). These results suggest that mechanical power output and pedal speed, a marker for muscle shortening velocity, are the main determinants of metabolic cost during submaximal cycling, whereas pedaling rate (i.e., activation-relaxation rate) does not significantly contribute to metabolic cost.

The purpose of this investigation was to determine the effects of cycle crank length on maximum cycling power, optimal pedaling rate, and optimal pedal speed, and to determine the optimal crank length to leg length ratio for maximal power production. Trained cyclists (n = 16) performed maximal inertial load cycle ergometry using crank lengths of 120, 145, 170, 195, and 220 mm. Maximum power ranged from a low of 1149 (20) W for the 220-mm cranks to a high of 1194 (21) W for the 145-mm cranks. Power produced with the 145- and 170-mm cranks was significantly (P < 0.05) greater than that produced with the 120- and 220-mm cranks. The optimal pedaling rate decreased significantly with increasing crank length, from 136 rpm for the 120-mm cranks to 110 rpm for the 220-mm cranks. Conversely, optimal pedal speed increased significantly with increasing crank length, from 1.71 m/s for the 120-mm cranks to 2.53 m/s for the 220-mm cranks. The crank length to leg length and crank length to tibia length ratios accounted for 20.5% and 21.1% of the variability in maximum power, respectively. The optimal crank length was 20% of leg length or 41% of tibia length. These data suggest that pedal speed (which constrains muscle shortening velocity) and pedaling rate (which affects muscle excitation state) exert distinct effects that influence muscular power during cycling. Even though maximum cycling power was significantly affected by crank length, use of the standard 170-mm length cranks should not substantially compromise maximum power in most adults.

i believe you can adapt to the crank length. it you take the circle path the foot travels, a 2.5mm difference in crank length only works out to be a few percent difference. not very much if you think about it. if you are a trackie and every rpm matters then short makes sense. for most of us it's not as big a deal.

different positions can be adapted to over time and still yeild good preformances. chris boardman is a great example of this, riding forward in track and hour records but still being able to ride road mountain stages in the daphine (sp?) etc with the top guys. i would view a 2.5mm change in crank arm less of a change the rotating your position forward. i may be wrong.

physiology papers are almost always inconclusive because it's hard to create a study were only one factor changes at a time. you also want to use strong athletes to predict what will happen to strong other athletes but these people don't do studies because they would have to drop their normal training.

lance should get them to make a tt bike exactly like his road bike. training is position specific. train at low cadences better at low cadences. same for forward positions or crank arm length. i wonder if lance said he didn't like the tt bike (because of the diffences) if he would be able to ride something else? trek pays him a tonne. i don't even think a highly sought after athlete like lance could even do that to trek.